than one molecule of ammonia, the numerical prefix is inserted between the names of the elements or radicals taking the place of hydrogen and the word amide or amine.

287. Chemical Formula.-The notation and use of symbols have been already explained (236). Chemical composition and reactions are expressed by writing them together; and such written expressions are called chemical formula. An empirical formula is one which states only what substances and what proportions of them or number of atoms form a compound; a rational formula aims to express the manner of atomic grouping, or the way the elements are combined. The empirical formula for alcohol is CHO, the rational formula is C,H,.OH, a compound of ethyl and hydroxyl. When the atoms of a group are more closely connected among themselves than with the other constituents of the compound, or when they play the part of a compound radical, they are separated from the rest by commas, inclosed in a parenthesis, or a single symbol (Cy), as in the case of cyanogen (C,N,), is substituted for the group. Thus the following equation may be written

=

=

The plus-sign (+) is also used to separate atomic groups. 288. Chemical Equations.-The results of chemical reaction are represented in the form of equations, which depend upon the principle that nothing is lost in the course of transformation. The bodies to be acted upon are placed at the left, and connected by the sign of addition +. The sign of equality signifies that the products of the change which are written at the right equal the bodies at the left. The equation also implies that the molecules at the left side are convertible into those written upon the right. The equation CaO + H,O= CaO,H, represents simply that, if a molecule of lime be added to a molecule of water, the product formed will be a molecule of calcic hydrate or slacked lime.

2

Symbol, H. Atomic Weight, 1; Quantivalence, 1; Specific Gravity, 1; Molecular Weight, 2; Molecular Volume, 2.

289. Its Position. In entering upon the description of the properties of chemical substances we begin with hydrogen, which is taken as the unit, or starting-point of the established system. It is an element of great importance in Nature, as well as in chemical theory; and is so individual in its character that it is difficult to classify with other elements, and so will be most conveniently considered at first and by itself.

290. History. It was known by Paracelsus, in the sixteenth century, that when iron is dissolved in sulphuric acid an air is given off; this air was shown by Boyle, in 1672, to be inflammable; and by Lemery, in 1700, to have detonating properties. But the first exact experiments upon it were made by Cavendish in 1766, and it was called by him inflammable air. In 1781 Cavendish made the great discovery that water is the sole product of the combustion of this gas, and Lavoisier therefore gave it the name

hydrogen, from two Greek words signifying water-gen

erator.

291. Occurrence in Nature.-Hydrogen is universally diffused, and takes an active and varied share in the chemical operations of Nature. Existing in water, which is decomposed with facility, it pervades the crust of the earth, and ministers to the transformation of minerals; while, as a large constituent of all living things, its changes contribute to carry on the processes of life. It is present in nearly all kinds of compounds, combined with other elements. It forms one-ninth of the weight of water, and the body which contains the largest proportion of it is hydric carbide (marsh-gas), of which it forms one-fourth. It was formerly held that hydrogen does not exist free in Nature; but it is now found uncombined in volcanic gases, in meteoric stones, and, as we have seen, exists free in immense masses in the atmospheres of the sun and stars.

FIG. 123.

Liberation of Hydrogen by Sodium.

292. Preparation.Hydrogen is generally obtained by decomposing water and setting the gas free. It is usuually collected in inverted jars filled with water, as represented in Fig. 123. If a bit of the metal sodium in a spoon be placed under the mouth of such a jar, it decomposes the water rapidly, combining with its

oxygen, and setting free the hydrogen, which rises in bubbles and displaces the water in the jar. Steam passed through a red-hot gun-barrel is decomposed by the iron,

which combines with the oxygen and sets the hydrogen free. A current of electricity passed through water severs its constituents, and liberates both oxygen and hydrogen, when they may be collected separately (141).

293. By the Use of Zinc.-Hydrogen is commonly prepared, however, by the action of dilute hydric sulphate (sulphuric acid) upon bits of zinc. The zinc is placed in a two

The

tube, with a funnel at top, admits the acid, when the action begins at once, the gas bubbles up freely and passes off through the curved tube, which delivers it under the mouth of an inverted jar, as before, but which now rests upon a support below the surface of the water. A vessel for the collection of gases, in this way, is called a pneumatic trough. It is usually a tank (represented in the cut as having glass sides), in which jars are filled with water, inverted, and then slid upon the shelf, the water being supported above its level by atmospheric pressure. When the jars are filled with gas they may be slipped off, mouth downward, into shallow vessels containing a little water, and kept for use. In the foregoing reaction the hydric

sulphate is decomposed by the zinc, while the hydrogen

is liberated, and zinc sulphate formed. The changes are represented by the following equation:

294. Chemical Properties.-As usually prepared, hydrogen has a disagreeable odor, arising from the impurities of the materials employed; but pure hydrogen is a transparent, tasteless, inodorous gas, very slightly soluble in water, inflammable, and having great chemical activity. It is an essential constituent of bases and acids, the latter being properly salts of hydrogen. It unites with metals and organic radicles, forming compounds called hydrides. It does not support respiration, and animals immersed in it soon die. When mixed with air it may be breathed without immediate injury, but from its tenuity it imparts a squeaking tone to the voice. All attempts to liquefy it, either by pressure or cold, have failed. In the gaseous state hydrogen is combined with itself, forming the molecule, H-H.

295. Its Lightness.-Hydrogen is the lightest of all known substances, being 14 times lighter than air, 16 times lighter than oxygen, and 11,000 times lighter than water. Hence it may be carried in jars with the open mouth downward, and transferred to other vessels by pouring upward. Soap-bubbles filled with it rise to the ceiling, and it gives the greatest levity to balloons; though they are usually inflated with a hydrocarbon gas, the lightness of which is due to the hydrogen it contains. Owing to the fineness of its molecules it will escape through the joints of apparatus that are perfectly tight to other gases; and a stream of it directed against one side of a piece of gold-leaf passes through so rapidly that it may be ignited on the other side.

296. Inflammability and Explosiveness.—If a jet of hy